[3.5][windows] bpo-27425: Add .gitattributes, fix Windows tests

[vstinner]

Mark binary files as binay in .gitattributes to not translate newline characters in Git repositories on Windows.

The git commit is not a cherry-pick, I simply copied the file from the master branch.

[vstinner]

This change should help to fix 3.5 buildbots like:
http://buildbot.python.org/all/builders/AMD64%20Windows10%203.5/

For bugs, see:

• http://bugs.python.org/issue27425
• http://bugs.python.org/issue29530

[vstinner]

Oh, I just saw that @zware proposed a change on .gitattributes in master: #840

Well, we can start with the current .gitattributes file to fix 3.5 buildbots, and then backporter further enhancements if needed.